This weekend Hollywood was again unable to lure teens to the multiplex in droves, as "Ender's Game" became the latest young adult adaptation to open with modest ticket sales.

The pricey sci-fi flick debuted with a so-so $28 million, according to an estimate from Lionsgate's Summit Entertainment, which distributed the movie. While the opening is far from disastrous -- it's actually $5 million more than industry predictions suggested -- it still isn't great given that the studio was likely hoping "Ender's Game" would launch a successful Y.A. franchise.

Meanwhile, in somewhat of a surprise, Johnny Knoxville's hidden-camera prank flick "Bad Grandpa" was the weekend's runner-up. After debuting at No. 1, the film saw its ticket sales tumble only 36% this weekend to $20.5 million, raising the picture's 10-day tally to $62.1 million.

Heading into the weekend, pre-release audience surveys indicated the 3-D animated movie "Free Birds" would easily top the comedy "Last Vegas." Instead, the latter film starring Morgan Freeman, Robert De Niro, Kevin Kline and Michael Douglas came in just ahead of the family film with $16.5 million -- a healthy start given its modest $28-million budget. "Free Birds," which cost $55 million to make, opened with $16.2 million.

Based on Orson Scott Card's bestselling 1985 novel, "Ender's Game" stars 16-year-old newcomer Asa Butterfield as a boy attempting to protect mankind from aliens. Though the movie is aimed at teenagers, it attracted an older crowd this weekend as 54% of the audience was over the age of 25. Those who saw the film assigned it an average grade of B+, according to market research firm CinemaScore. Even if the movie generates positive word-of-mouth, it will still have to contend next weekend with the sequel "Thor: The Dark World," which is expected to be a big hit.

"The real key to the success of the movie is going to be how we hold next weekend," acknowledged Richie Fay, Lionsgate's president of domestic distribution. "But if we can survive against 'Thor,' I think we’ll be in good shape."

The movie is the latest in a series of films made for the under-18 crowd that have failed to connect at the box office. In the wake of the massively popular "Twilight" and "Harry Potter" franchises, "The Hunger Games" has been one of the only young adult novel adaptations to work at the multiplex. (The next entry in the "Hunger Games" series, "Catching Fire," is set to hit theaters with a possible $150 million opening on Nov. 22.) This summer, the second installment in the "Percy Jackson" series grossed just $67.3 million in the U.S. and Canada -- roughly $20 million less than the first film collected domestically in 2010. Other Y.A. casualties have included "The Mortal Instruments: City of Bones," "The Host" and "Beautiful Creatures," none of which ultimately made it above $32 million.

"Ender's Game" cost a pretty penny to produce -- about $110 million, co-financed by OddLot Entertainment, the visual effects company Digital Domain and Summit. However, the financiers insist that international pre-sales took care of the majority of the film's production costs, meaning they would not have much on the line financially.

While "Ender's Game" played to an older-than-expected demographic, "Last Vegas" resonated with exactly the crowd distributor CBS Films anticipated: the AARP set. About 83% of moviegoers were over the age of 25, and 47% were over 50. Though critics loathed the movie, filmgoers enjoyed it, giving the film an average A- grade.

Financed by CBS and Good Universe, "Last Vegas" follows four older gentlemen who travel to Sin City for a bachelor's party. The movie played best in the South -- where Freeman’s movies are often popular -- and the Midwest: On Saturday, the movie sold more tickets at a theater in Oklahoma City than any other venue in the country.

“We always thought this would be a fly-over movie because movies about Vegas -- which is a destination holiday location -- tend to do better in middle America,” said Steven Friedlander, executive vice president of theatrical distribution for CBS Films.

As for "Free Birds," the movie is the first partnership between Relativity Media and Reel FX Animation Studios. The movie is the first in a slate of planned low-budget animated films to come from the two companies.

The movie follows two turkeys (voiced by Woody Harrelson and Owen Wilson) who are attempting to save their kind from ending up as Thanksgiving dinner. Given the film's subject matter, Relativity is hopeful the movie will play well over Turkey Day. Though the movie didn't get off to a fantastic start, it earned an average grade of A- from moviegoers, and no other films for little kids are slated to be released until Nov. 27.